Audi S4 Heater Failure Linked to Daylight Sensor Issue

Having some experience working on Audis, I faced a similar issue with my 2006 Audi A4 2.0T. The symptoms you describe match what I encountered, reduced heating performance and an error code related to the HVAC sensor system. After multiple attempts to diagnose it myself, I took it to a workshop. They found that the temperature sensor wiring harness had developed a poor connection due to normal wear and movement. The solar sensor was actually fine, but the faulty connection was causing incorrect readings. The fix was straightforward, they repaired the damaged wire connections and checked all related plugs in the HVAC sensor circuit. Total cost was 95 Euro including diagnostic time and parts. To better assist with your specific case, could you share: Which error code is stored? Does the heating work at all or just poorly? Have you noticed any changes in the automatic climate control behavior? Are there any other warning lights on the dash? These details would help determine if your issue matches what I experienced with my A4, as sensor fault patterns can vary between different engine and trim configurations.

Latest service was done at 140064 KM. After checking the error codes again, I noticed the OBD scanner shows P0116. I should mention the heating still works but very weakly, especially on the passenger side. The automatic climate control seems erratic, sometimes it blasts hot air then suddenly goes cold. No other warning lights are showing on the dash. The temperature sensor seems to be the main culprit based on the behavior, but I want to be sure before replacing parts. I had considered doing sensor calibration myself but would rather get a professional opinion first since multiple HVAC sensor components could be involved. Just want to make sure I address the root cause rather than chase symptoms.

Based on your updated info and error code P0116, this definitely points to an engine coolant temperature sensor issue rather than the solar sensor I initially suspected on my A4. The erratic temperature behavior you describe matches what I experienced before the full repair. In my case, the sensor itself was failing intermittently, causing similar symptoms, weak heat output and inconsistent climate control operation. The passenger side being colder is a classic sign of coolant temperature sensor problems, as the system cant properly regulate the temperature split. The repair on my A4 involved: Replacing the coolant temperature sensor (89 Euro for parts), Checking coolant level and bleeding the system, Clearing the error codes, Verifying proper sensor operation Total repair time was about 2 hours. Much simpler than dealing with solar sensor or climate control module issues, which I initially feared. The temperature sensor replacement resolved all symptoms, including the erratic automatic climate control behavior. Given your identical symptoms and error code, I would recommend focusing on the coolant temperature sensor rather than pursuing sensor calibration or other HVAC components. The P0116 code specifically indicates a performance problem with the engine coolant temperature sensor circuit.

Thanks for the detailed response. You're right that the P0116 points to the coolant temperature sensor rather than my initial solar sensor theory. The identical symptoms you described with your A4 are encouraging, as it suggests a straightforward fix rather than a complex HVAC sensor system issue. I'll schedule an appointment to have the coolant temperature sensor replaced, which seems much more reasonable at around 89 Euro than what I feared might be an extensive climate control repair. The weak passenger side heat and erratic temperature behavior match perfectly with what you experienced. I appreciate you sharing the complete repair process. Knowing they'll need to check the coolant level and perform system bleeding helps me understand the full scope. It's reassuring that sensor calibration or other HVAC sensor repairs weren't necessary in your case. I'll post an update once the repair is complete to confirm if the temperature sensor replacement resolves my issues too. This should help others who encounter similar symptoms with their S4s in the future.